Aspartate kinase catalyzes a first step in the biosynthesis from Asp of Lys (and its precursor diaminopimelate), Met, and Thr. In E. coli, a distinct isozyme is inhibited by each of the three amino acid products. The Met-sensitive (I) and Thr-sensitive (II) forms are bifunctional enzymes fused to homoserine dehydrogenases and form homotetramers, while the Lys-sensitive form (III) is a monofunctional homodimer.
The Lys-sensitive enzyme of Bacillus subtilis resembles the E. coli form but is an alpha 2/beta 2 heterotetramer, where the beta subunit is translated from an in-phase alternative initiator at Met-246. This may be a feature of a number of closely related forms, including a paralog from B. subtilis.